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Single Beam Acoustic Depth Measurement Techniques


Quiz Questions

1.Acoustic depth sounding was first used in the Corps back in the 1960s but did not replace reliance on lead line depth measurement until the 1970s or 1980s.
True
False
2. Acoustic depth measurement systems measure the elapsed time that an acoustic pulse takes to travel from a generating transducer to the waterway bottom and back.
True
False
3.Depth corrected to referenced water surface d = ½ (v • t) + k + dr ,what does k represent here?
System index constant
Measured elapsed time
Average velocity of sound
4.Determining the sound velocity, v, is perhaps the most critical factor in using acoustic depth sounders.
True
False
5.The sound velocity does not vary with the density and elastic properties of the water.
True
False
6.The index constant does not contain any electrical and/or mechanical delays inherent in the measuring system, including return signal threshold detection variations.
True
False
7.The transducer draft and index constant must not be applied to the reduced time distance to obtain the corrected depth from the reference water surface.
True
False
8.The accuracy of the absolute time measurement generally varies with ______________.
Distance
Depth
Velocity
9.__________________ are measured at the – 3 dB half-power points.
Bandwidths
Sensitivities
10.Higher frequency transducers have a frequency range of _______________________.
50 kHz to 800 kHz
100 kHz to 1000 kHz
100 kHz to 800 kHz
11.Lower frequency transducers tend to have __________________.
Smaller beam widths
Larger beam widths
12.The _______________ transducers provide more precise depth measurement.
Lower frequency
Higher frequency
13.The approximate bottom footprint size of a transducer can be computed by the following equation:
Linear coverage (ft) = 2 • D • tan (a/2)
Linear coverage (ft) =3.14 • D 2 • tan2 (a/2)
14.The wider the beam, the less effect vessel roll or pitch will have since the transducer beam width falls within the vertical.
True
False
15.The most commonly employed transducer frequency in USACE river and harbor navigation projects is______________________.
100-108k Hz
200-208 kHz
300-308 kHz
16.Figure9-4 represents ___________________________________.
Typical single beam echo sounders used in Corps
Raytheon DE 719 analog-recording portable echo sounder
17.The Hydrotrac is a single frequency, Recorder/Digitizer/Transceiver and is a highly integrated digital and analog sounder packaged into a small, waterproof housing.
True
False
18._____________surveys are run either normal to (i.e., cross-sectioned) or longitudinal with the channel alignment.
50 and 200 ft
20 and 100 ft
19._____________surveys are run either normal to (i.e., cross-sectioned) or longitudinal with the channel alignment.
Single beam
Double beam
20.________________echo sounders typically collect depth data at a rate of 5 to 20 soundings per second.
Single beam
Double beam
21.An approximate computation of the update rate can be made from the following equation:
Update rate (milliseconds) = 1185 • ( D / v ) • tan(a/2) where
D is the average or project depth
V is the velocity in knots
a is the transducer beam width
All of the above
None of the above
22.Many districts have now incorporated motion compensation into single beam systems. Since vessel roll, pitch, yaw, and heave conditions can occur simultaneously and at different periods, either visual or automated interpretation of a single beam analog profile record to reduce these errors is an imprecise process, at best.
True
False
23.To best minimize the adverse effects of vessel motion, single beam systems used for dredging and navigation surveys in rough sea states should be equipped with automated heave sensors, and also pitch and roll sensors.
True
False
24.Excessive _________________ can inject position errors in the measured depth.
Roll & pitch
Pitch & heave
Heave & roll
25.The important characteristic of pitch offset is that the along-track displacement caused by pitch offset is _____________________to water depth.
Directly proportional
Inversely proportional